memchr.c

/* Copyright (C) 1991, 1993, 1996-1997, 1999-2000, 2003-2004, 2006, 2008-2010 Free Software Foundation, Inc. Based on strlen implementation by Torbjorn Granlund (tege@sics.se), with help from Dan Sahlin (dan@sics.se) and commentary by Jim Blandy (jimb@ai.mit.edu); adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), and implemented by Roland McGrath (roland@ai.mit.edu).NOTE: The canonical source of this file is maintained with the GNU C Library.Bugs can be reported to bug-glibc@prep.ai.mit.edu.This program is free software: you can redistribute it and/or modify itunder the terms of the GNU General Public License as published by theFree Software Foundation; either version 3 of the License, or anylater version.This program is distributed in the hope that it will be useful,but WITHOUT ANY WARRANTY; without even the implied warranty ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See theGNU General Public License for more details.You should have received a copy of the GNU General Public Licensealong with this program. If not, see <http://www.gnu.org/licenses/>. */#ifndef _LIBC# include <config.h>#endif#include <string.h>#include <stddef.h>#if defined _LIBC# include <memcopy.h>#else# define reg_char char#endif#include <limits.h>#if HAVE_BP_SYM_H || defined _LIBC# include <bp-sym.h>#else# define BP_SYM(sym) sym#endif#undef __memchr#ifdef _LIBC# undef memchr#endif#ifndef weak_alias# define __memchr memchr#endif/* Search no more than N bytes of S for C. */void *
__memchr (voidconst *s, int c_in, size_t n)
{
/* On 32-bit hardware, choosing longword to be a 32-bit unsigned long instead of a 64-bit uintmax_t tends to give better performance. On 64-bit hardware, unsigned long is generally 64 bits already. Change this typedef to experiment with performance. */typedefunsignedlongint longword;
constunsignedchar *char_ptr;
const longword *longword_ptr;
longword repeated_one;
longword repeated_c;
unsigned reg_char c;
c = (unsigned char) c_in;
/* Handle the first few bytes by reading one byte at a time. Do this until CHAR_PTR is aligned on a longword boundary. */for (char_ptr = (constunsignedchar *) s;
n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
--n, ++char_ptr)
if (*char_ptr == c)
return (void *) char_ptr;
longword_ptr = (const longword *) char_ptr;
/* All these elucidatory comments refer to 4-byte longwords, but the theory applies equally well to any size longwords. *//* Compute auxiliary longword values: repeated_one is a value which has a 1 in every byte. repeated_c has c in every byte. */
repeated_one = 0x01010101;
repeated_c = c | (c << 8);
repeated_c |= repeated_c << 16;
if (0xffffffffU < (longword) -1)
{
repeated_one |= repeated_one << 31 << 1;
repeated_c |= repeated_c << 31 << 1;
if (8 < sizeof (longword))
{
size_t i;
for (i = 64; i < sizeof (longword) * 8; i *= 2)
{
repeated_one |= repeated_one << i;
repeated_c |= repeated_c << i;
}
}
}
/* Instead of the traditional loop which tests each byte, we will test a longword at a time. The tricky part is testing if *any of the four* bytes in the longword in question are equal to c. We first use an xor with repeated_c. This reduces the task to testing whether *any of the four* bytes in longword1 is zero. We compute tmp = ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7). That is, we perform the following operations: 1. Subtract repeated_one. 2. & ~longword1. 3. & a mask consisting of 0x80 in every byte. Consider what happens in each byte: - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff, and step 3 transforms it into 0x80. A carry can also be propagated to more significant bytes. - If a byte of longword1 is nonzero, let its lowest 1 bit be at position k (0 <= k <= 7); so the lowest k bits are 0. After step 1, the byte ends in a single bit of value 0 and k bits of value 1. After step 2, the result is just k bits of value 1: 2^k - 1. After step 3, the result is 0. And no carry is produced. So, if longword1 has only non-zero bytes, tmp is zero. Whereas if longword1 has a zero byte, call j the position of the least significant zero byte. Then the result has a zero at positions 0, ..., j-1 and a 0x80 at position j. We cannot predict the result at the more significant bytes (positions j+1..3), but it does not matter since we already have a non-zero bit at position 8*j+7. So, the test whether any byte in longword1 is zero is equivalent to testing whether tmp is nonzero. */while (n >= sizeof (longword))
{
longword longword1 = *longword_ptr ^ repeated_c;
if ((((longword1 - repeated_one) & ~longword1)
& (repeated_one << 7)) != 0)
break;
longword_ptr++;
n -= sizeof (longword);
}
char_ptr = (constunsignedchar *) longword_ptr;
/* At this point, we know that either n < sizeof (longword), or one of the sizeof (longword) bytes starting at char_ptr is == c. On little-endian machines, we could determine the first such byte without any further memory accesses, just by looking at the tmp result from the last loop iteration. But this does not work on big-endian machines. Choose code that works in both cases. */for (; n > 0; --n, ++char_ptr)
{
if (*char_ptr == c)
return (void *) char_ptr;
}
return NULL;
}
#ifdef weak_aliasweak_alias (__memchr, BP_SYM (memchr))
#endif